Early attempts at mounting and connecting integrated circuits and semiconductor chips to printed wiring boards (PWBs) frequently resulted in unreliable connections. More particularly, attempts at connecting commercial-off-the-shelf (“COTS”) large-size, high-pin-count (“HPC”) plastic ball grid arrays (PBGAs) to PWBs included additional difficulties during attachment and use due certain characteristics to the PBGAs. Some of the COTS PBGAs display warpage and non-coplanarity, which may cause shorting and/or structural weaknesses after soldering. These difficulties may be enhanced or exaggerated when the PWBs are exposed to harsh environments, such as environments having thermal cycling and mechanical vibrations.
Non-coplanarity in a PBGA is measured by the largest distance between a given ball and a theoretical plane formed by three highest balls. Non-coplanarity may be observed by placing a PBGA on a flat plate or surface where only three balls make contact with the flat plate. The non-coplanarity prevents other balls from contacting the surface, leaving a gap. Non-coplanarity may, however, change during thermal cycling, depending in some cases on the thermal coefficient of expansion mismatch of the materials used in the PBGA, such as silicon, substrate, encapsulant, and die adhesive and structural inconsistencies.
In some cases, the COTS PBGAs, “as received” from the manufacturer, include non-coplanarity characteristics even at room temperature. COTS components, despite following commercial standards, may display deviations of 0.004 inch for a small-size COTS PBGA, which for 0.020 inch diameter solder balls indicates a 0.004/0.020 deviation or 20% non-coplanarity. For small-size COTS PBGA, this deviation, equaling approximately ⅕th of the height of the joint, results in a weak joint between corresponding contacts on the PBGA and the PWB. For large-size COTS PBGA, the problem may be exaggerated with the deviation from coplanarity exceeding 0.006 in some cases. The PGBAs may experience additional warpage during soldering when localized temperatures may increase dramatically. For example, the soldering temperature may range from about 215° C. to about 220° C. for Tin63%:Lead37% solder and about 245° C. to about 250° C. for lead-free Tin:Silver:Copper (SAC) alloy. At these soldering temperatures, previously planar PBGAs may experience new warpage and originally warped PBGAs may further deform, exacerbating the issue.
FIG. 1A shows an example of a PBGA prior to soldering to a PWB. As shown in FIG. 1, a PBGA 10 may include an encapsulant 11, a substrate 12 and solder balls 13. Although the PBGA 10 may be coplanar at room temperature when received from the manufacturer, the PBGA may warp during soldering when localized temperatures increase. As shown in FIG. 1A, for most COTS PBGA the encapsulant 11 does not extend across the entire upper surface of the substrate 12. As such, the substrate 12 has been shown to experience localized warpage of the outer portions of the substrate 12 without the encapsulant 11 during soldering of the solder balls 13.
FIG. 1B shows the effects of warpage on the quality of the solder joints. As shown, the PBGA 10 may warp during or after soldering, especially at the edges of the PBGA 10, with the substrate 12 departing from coplanarity. For example in FIG. 1B, at the edges of the PBGA 10, a distinctive downward curve is shown. This downward warpage may cause shorts in the electrical circuits if the solder balls 13 flatten and touch one another as shown in FIG. 1 when the PBGA is soldered to the PWB 30. Alternatively, if the PBGA is warped with an upward curve (not shown in the figures), the solder ball may fail to make contact with the PWB, resulting in a gap or open circuit.
If a warped or non-coplanar PBGA is received from the manufacturer and soldered to a PWB in the normal process, the warpage may be exaggerated during the soldering process and compound the problem. In COTS HPC PBGA, the warpage due to the soldering process may be difficult to predict due to several variables associated with the materials and processes. For example, the PBGA typically warps at the usual 215° C. to 220° C. soldering temperature. However, even when cooling from about 183° C., the temperature at which typical solder solidifies, to room temperature, the PBGA may continue to warp, inducing stress into the solder joints. Further, repeated thermal cycling, such as for example from −40° C. to +100° C., during actual use may induce additional warping in the PBGA and result in stressed solder joints due to repeated warping and unwarping actions.
Even if the induced warpage or non-coplanarity is not sufficient to cause a short or gap, a non-coplanar PBGA may induce greater stress and strain on the solder joints used to attach the PBGAs to the PWBs. Typically, PBGAs and PWBs have different thermal coefficients of expansion (TCE), resulting in problems when exposed to harsh thermally cyclic environments and vibration. When warped or non-coplanar PBGAs experience thermal expansion and contraction, the individual solder joints the PBGA to the PWB often experience different amounts of stress and strain. Stresses induced by thermal cycling, vibration and shock during use limit the life of the solder joint and the reliability of the system. Highly stressed, weak solder joints inherently include a lower life expectancy. Over time, degradation and cracking results at the solder joints from temperature cycling, especially where localized warping of the PBGA has already induced stress on the solder joints. As a result, the thermal expansion and contraction experienced by the solder joints increases with the amount of warpage of the PBGA, resulting in possible mechanical and electrical failure over time.
Previous attempts to connect certain components to PWBs have included the use of compliant leaded interposers. Such compliant leaded interposers have included etched leads, formed from thin copper leaf, which directly connect the electrical contacts on lightweight components, such as chip scale packages, to the electrical contacts on the PWB. An example of such a compliant leaded interposer is taught in U.S. Pat. No. 6,830,177. However, the thin etched leads of previously taught interposers are incapable of supporting larger components, such are large area grid array components. More particularly, the previously taught interposers fail to provide sufficient mechanical support and robustness for heavy PBGA components used in may advance electronic packages.
Therefore, it would be desirable to compensate for warpage or non-coplanarity in PBGA and to provide a robust mechanical and electrical connection between the PBGA and the PWB after soldering.